Chemical storage vessels such as liquid hydrogen storage tanks are protected from overpressurization by relief devices. The relief device is normally a control valve that is set to open when internal pressure in the storage vessel reaches a predetermined level. When the valve is opened, material venting from the tank is normally conducted through a vent pipe which is arranged to discharge the venting gas into the air or ambient atmosphere at levels between 12 and 60 feet above grade in the case of a hydrogen storage tank. In the case of storage tanks for normally liquefied gases such as liquid hydrogen, the storage vessels range in size where the inner vessel can contain between 1000 and 25,000 gallons of liquid hydrogen. Normally relief valves on these tanks are set to discharge at 150 psig at 700.degree. F. In addition to the relief valves there are primary rupture discs which are set to relieve at 180 psi g at 70.degree. F. and secondary rupture discs set to relieve at 225 psig at 70.degree. F. The primary rupture discs may discharge into the same vent pipe that the primary relief valves direct the venting gas toward. The secondary discs discharge into a separate vent pipe, directed vertically upward and normally covered by a sealed cap. Normally the vent pipe or stack terminates substantially vertically upward at a distance of 12 to 60 feet above grade on which the tank or storage device is installed so the venting gas is directed substantially vertically upward. In the case of hydrogen, a flammable gas, if ignited, could constitute a hazard to personnel and equipment in the immediate vicinity of the storage vessel. An upwardly directed vent pipe left uncovered in ambient atmosphere and exposed to everyday weather conditions could fill with rain water, which in turn could freeze, thus plugging the vent pipe. Since vent pipes associated with secondary rupture discs do not have valves which could leak and because the burst pressure is greater than the operating pressure, these discs seldom rupture prematurely and thus the vent pipe associated with these discs may be protected from the elements with a sealed cap.
In the past, primary vent pipe closures have used hinged flappers which have had the problem of shearing the hinge pin when the vent was operated or tested thus exposing the vent stack to atmospheric conditions. Branch conduits with downwardly projecting openings have been used, but these would direct the venting gas downwardly instead of upwardly into the ambient atmosphere. Various other devices have been proposed, including internal baffles on vertical uncapped pipes to drain rain or snow while discharging gas vertically upward, and redirecting downwardly exhausted gas from any side pipes upwardly.